1. Field of the Invention
The present invention relates to reclaimed rubber obtained by devulcanization treatment of vulcanized rubber from discarded tires, etc. for their recycling and to a vulcanized rubber reclaiming process for recycling of vulcanized rubber from discarded tires, etc. The reclaimed rubber is supplied for reuse in tires or for use as a modifying material for asphalt or the like.
2. Description of the Related Art
Various processes have been proposed in the past for reclaiming vulcanized rubber from discarded tires, etc. by destruction of its crosslinked structure, to allow its use for the same purposes as unvulcanized rubber. For example the PAN process, which is widely employed as a reclaim process for vulcanized rubber in Japan, accomplishes devulcanization treatment in a few hours under high pressure vapor. The reclamator process, which is a continuous reclaim process using a single-screw extruder, is also known as a continuous devulcanization treatment by shear force. Examples of the latest known techniques for reclaiming devulcanization of vulcanized rubber by such shear force and heat are disclosed in, for example, Japanese Unexamined Patent Publication No. 9-27724 and Japanese Unexamined Patent Publication No. 10-76001.
Other vulcanized rubber reclaim processes that have been proposed include high-speed mixing devulcanization, microwave devulcanization (SiR, FiX; Elastomerics, 112(2), 38(1980), Japanese Examined Patent Publication No. 2-18696), radiation devulcanization, ultrasonic devulcanization and the like, and some of these are actually being employed.
Thus, a number of processes for reclaiming of vulcanized rubber have been proposed, but none of them can be said to be fully satisfactory processes. For example, although the PAN process has been used for tire materials made primarily of natural rubber (NR) or styrene-butadiene rubber (SBR) for reasons of performance and cost, even the PAN process is insufficient from the standpoints of performance and cost, and therefore the extent of its use is still limited.
In the reclamator process and the aforementioned shear force-utilizing reclaim processes described in Japanese Unexamined Patent Publications No. 9-227724 and No. 10-176001, which are continuous treatment techniques with cost advantages, the treatment capacity has not always been adequate. The other heretofore proposed reclaim processes have also been inadequate in terms of both cost and treatment capacity.
Reclaimed rubber obtained by the aforementioned treatment processes has had the problem of inadequate performance, and therefore it cannot be said, at the current time, that a sufficient degree of research has been conducted on the performance of reclaimed rubber.
It is therefore an object of the present invention to provide reclaimed rubber with consistently excellent performance among reclaimed rubber obtained by devulcanization treatment.
It is another object of the invention to provide a vulcanized rubber reclaiming process that is a continuous process with cost advantages and has a high treatment capacity.
As a result of diligent research focused on the proportion of gel and sol in reclaimed rubber in an attempt to overcome the problems referred to above, the present inventors have found that the aforementioned object can be attained by limiting the proportion of sol to gel, etc. to within a specified range, and have thus reached completion of the reclaimed rubber of the present invention. Furthermore, as a result of diligent research on the conditions of continuous reclaim with twin-screw extruders, the present inventors have found that the other aforementioned object can be achieved when ring-shaped segments of a specific construction are formed in the screw structure of a twin-screw extruder, and have thus reached completion of the vulcanized rubber reclaim process of the present invention.
According to the invention, there is the provision of reclaimed rubber obtained by devulcanization treatment of vulcanized rubber, wherein the proportion of sol to gel is 10-80%, the weight-average molecular weight (Mw) of the sol by gel permeation chromatography (GPC) is within the range of 20,000-300,000, and the degree of swelling of the gel is 3.0-20.0.
In the reclaimed rubber according to the invention, the above-mentioned proportion is preferably 20-60%, and the weight-average molecular weight (Mw) of the sol by gel permeation chromatography (GPC) is preferably within the range of 30,000-250,000. Also, the degree of swelling of the gel is preferably 4.0-15.0.
Further, according to the invention, there is the provision of a process for reclaiming vulcanized rubber, a twin-screw extruder having a pair of screws being used for devulcanization treatment of vulcanized rubber, wherein at least one pair of mutually interlocking ring-shaped segments is provided at an appropriate location of the pair of screws in the twin-screw extruder.
According to the process of the invention, the rotation rate of the screw in the twin-screw extruder is preferably 20-300 rpm, and the temperature of the cylinder (barrel) at the section of maximum temperature is preferably 100-300xc2x0 C.